Power efficiency is of paramount importance in mobile devices. To maximize power efficiency, mobile devices utilize high efficiency switching circuitry. High-efficiency operation of switching circuits requires low on-state switch resistance. However, low on-state switch resistance may generate a large output current in the event the output terminal of the switching circuit is short-circuited. Large output currents may cause damage to the switching circuit. Therefore, there is a need to develop a short-circuit protection circuit to be utilized in high-efficiency switching output stage circuits to prevent circuit damage.
Different short-circuit protection circuits may be utilized; however these solution have significant downsides. For example, a protection circuit may be added in series with the output transistor. While this short-circuit protection circuit may prevent short-circuit currents, the addition of a series circuit with the output creates an increase in power dissipation and thus reduces the overall efficiency of the circuit.
Traditionally, digital output stages are switched, while analog output stages are controlled in a continuous fashion. However, there are cases in which it is advantageous to switch analog output stages.
FIG. 1 shows an example of a typical analog switched output stage 100. Switched output stage 100 includes a driver 102 and a complementary pair of output transistors M2 104 and M1 106 configured to drive a load circuit (not shown) that couples to output terminal OUT. An example load circuit is a loudspeaker. Output transistors 104, 106 may, for example, be bipolar junction transistors (BJTs) or metal-oxide field effect transistors (MOSFETs).
It is desirable for a non-switched output stage to be able to accurately establish the current, or the voltage, applied to the load circuit. A known technique to establish accurate analog output currents (or voltages) is to measure the output current (or voltage) at the output terminal OUT, and adjust the gate voltages of the output stage transistors through a feedback loop, until the measured output current (or voltage) reaches a desired value. Because output transistors such as M1, M2 (106, 104) dissipate a significant amount of power—reducing the power delivered to the load—the efficiency of the non-switched output stage is ultimately also reduced.
In a switched output configuration, transistor gate voltages are not accurately adjusted. Rather, the output transistors gate voltages are toggled between a high voltage and low voltage. In case of a NMOS transistor, the transistor is in an on state when the gate voltage is high. Conversely, when a NMOS transistor gate voltage is low the transistor is in an off state. In case of a PMOS transistor, the transistor is in an off state when the gate voltage is high. Conversely, when a PMOS transistor gate is low, the transistor is in an on state. In the on state, the resistance between the drain and the source terminals of the transistor is minimized. This reduction in resistance results in a low voltage across the transistor terminals. The low resistance and low voltage results in low power dissipation in the transistor. The relationship between voltage, current, resistance and power dissipation can be expressed as follows:P=VI=I2R  Eq. (1)
In the off state, the resistance between the drain and source terminals of the transistor is maximized. The increase in resistance results in a decrease in current flowing through the transistor. The reduction of transistor current results in low power dissipation. The low power dissipation characteristic during on state and off state of a transistor allow switching circuits to accomplish very low power dissipation and subsequently high power efficiency. An example of a switching circuit is a class-D circuit.
Protecting the stage against short-circuits may be accomplished by measuring and reducing the output current when a given threshold value is observed. A circuit capable of measuring the output current may be coupled in series with the circuit output. However, a series circuit configuration will result in the dissipation of additional power, which would otherwise be delivered to the load circuit. Therefore a series coupled solution will reduce the efficiency of the output stage. Alternatively, coupling a measuring circuit in parallel in order to measure the output current has the drawback that the same circuit does not provide any reduction in the power delivered to the load circuit, and hence cannot protect against a short-circuit condition.
FIG. 2 shows an example of a typical switched output stage 200 with short-circuit protection. Switched output stage 200 includes a driver 203 coupled via short-circuit protection circuit 201 to an output transistor 204. Short-circuit protection circuit 201 includes a reference transistor M1a 202, the gate of which is coupled to the gate of output transistor M1 204 and the output of driver 203. The gate terminals of transistors M1a 202 and M1 204 are coupled to the driver output terminal. The source terminals of transistors M1a 202 and M1 204 are coupled to ground. Transistor M1a 202 is a scaled version of output transistor M1 204. In saturation region, the current I1a flowing through transistor M1a 202 is a scaled version of current I1 flowing through output transistor M1 204. Current limiting circuitry 205 is coupled to the drain of reference transistor M1a to limit the current through reference transistor M1a 202 and output transistor M1 204.
The relationship between the output current I1 through transistor M1 204 and the reference current I1a through reference transistor M1a 202 may be expressed as:
                              I                      1            ⁢            a                          =                              I            1                    ⁡                      (                                                            W                                      1                    ⁢                    a                                                  ⁢                                  L                  1                                                                              W                  1                                ⁢                                  L                                      1                    ⁢                    a                                                                        )                                              Eq        .                                  ⁢                  (          2          )                    
When the device is not operating in the saturation region, the current I1a flowing through transistor M1a 202 cannot be accurately scaled to the current I1 flowing through transistor M1 204. Hence when not operating in the saturation region, the transistor current is instead highly dependent upon the voltage applied across the drain and source terminals of the transistor.
For analog switched output stages, such as may be utilized with class-D stages and capacitive charge pump circuits, where the switching output transistors are not configured to only operate in saturation region, improved short-circuit protection is desired.